I like many others have read Shane Actons book "Shrimpy - Around the World in an 18ft Yacht" which of course would give anyone an appetite for long distance cruising. I would sail the Oceans no problem but not in an 18ft Yacht.
I'm currently finishing off a Sailwind 27 the previous owner sailed the same model from Scotland to Florida at ease and I have total faith in it. However if you look at the list for the ARC there is not a boat under 30ft.
What in your opinion is the smallest size monohull you'd cross an ocean with?

That's the smallest I would consider for reasons of comfort. The smaller you go, the rougher the ride and the more spartan the experience. Lots of trips have been done with smaller vessels in safety, but not with much comfort.

Serge Testa did his circumnavigation in 500 days in Acrohc Australis which is 11 feet 10 inches long. It was a deep keel spartan yacht that he constructed by himself in Australia. You can see it on display in a museum in Brisbane.

You can also read his book, "500 Days - Around the World on a 12 Foot Yacht". Serge Testa. Trident Press.

This is a simple question with a million answers. It really depends on what YOU want. If you want a boat to take you alone from the Canaries to the Caribbean then almost anything seaworthy will do; a Swedish acquaintance of mine crossed last year in a tiny boat less than 20 ft. long. Even a couple can do it on a tiny boat as another Swede proved some years ago (IIRC the different designs were named "Bris"). Comfort is a very relative thing at sea, and in that regard displacement is probably more important than length; light boats tend to have faster motions. Most of the boats mentioned here are on the heavy side, which usually means sturdier as well.

A big question is this: what are you going to do once you get there? My Swedish friend rather quickly shipped his boat back to Europe; his goal was simply to make the trip. The boat was completely unsuited for living aboard. The 28' Bristol Channel Cutter, the 27' NorSea, and the 26' Contessa all provide a basic level of comfort for cruising the islands. The 22' Falmouth Cutter is a pretty neat design but has very limited space and privacy, although I would consider it seaworthy enough.

Many cruisers have looked at my 31' boat and thought it impossibly small, whereas many others thought it was quite nice for the purpose. Much depends on your attitude. To determine the minimum for you first I think you should consider how many people and how much equipment you intend to take along, then limit the search to boats that have enough capacity (not the number of berths!) If you plan on picking up crew somewhere then I can tell you from personal experience that it is harder to find crew for smaller boats; anything under 30' will not be very attractive.

As for the ARC the reasons there are no small boats is simple: the required crew and equipment pretty much demand a minimum of 30 feet. Specifically, the boat has to have at least two crew, liferaft, HF or satellite radios, and a long list of other gear. And the ARC folks need to be convinced that the boat meets their safety standards before it can join. Add in the high cost to join the ARC. And then consider the nature of those attracted to the social and competitive aspects of the ARC contrasted with the type of person that typically wants to sail a really small boat. In the interests of full disclosure: I am not a fan of the ARC, but I do appreciate that it takes a lot of boats to St Lucia so the rest of us can experience an uncrowded Barbados, Tobago and Trinidad

You do not see boats below 30' on ARC because ARC has a size limit. Not because anything under X feet is too small. ARC starts from Las Palmas and goes to Santa Lucia. You can do this in a kayak, a tub or a barrel. People row this stretch and this year there will be a swim attempt.

One can sail across some parts of the Ocean in almost anything. It is less comfortable (always), less safe (often), but not impossible. It has been proven by countless brave persons.

But sailing across an ocean in the good season, in the tropics, in the tradewind zones should NOT be mixed up with other sports and a boat capable of the former may, or may not be, capable of more demanding passages. Again, it does not mean they cannot be made or attempted. They will simply be even less comfortable, safe, up to the limit of plain accident waiting to happen.

Then, the human factor cannot be left out of the equation - a good sailor will probably be able to sail safely in a much smaller boat than a less experienced person.

We have sailed around in a 26 footer and we met many (very many) smaller boats attempting the same.

From this perspective I will say a 25 LWL and 3000 kilograms of light displacement (mid-displ. hull, long keel) are just on the edge of comfort and safety. A slightly bigger boat could probably be much safer and maybe also a bit more comfortable.

Our ocean experience was zero when we took off. Now I would rank it say advanced beginner, it is 7 years down the road and 35k+ miles on.

There is a range of really small boats (e.g. the Dutch Midgets, Swedish Havsfidra, etc) which are about 20' long and very conservatively designed. I would say this size is an absolute minimum for a solo sailor on the easiest of round-the-world routes. Anything below this size should be labeled as extreme sailing ;-)

John Guzzwells Trekka at 20ft and very light displacement made not 1 but 2 circumnavigations,the first solo and the second doublehanded, no drama, just good seamanlike voyages so this notion that you need heavy displacement is nonsense,of course it was probably easier back in the 1950s and 60s as there wasnt all the peer pressure to carry thousands of pounds of stuff you dont need. Trekka and her siblings at 20ft were very small light boats but still capable of carrying everything their crews required. I would think they would be about as small as practical,there are lots of suitable boats in this size range such as the old Cal 20, certainly a bit small for most of us though.
Steve.

I pretty much knew that it was a question with no solid answer I just wanted to get an idea from the experience and views of others. I have seen some 21 footers I'd feel safe enough in and some bigger monohulls I wouldn't take outside the bay.
The Sailwind 27 with a 10ft beam seem plenty big enough to me but when I mention the Scotland to Florida trip it is sometimes frowned upon. Sure I'd cross the Atlantic with it and the Pacific too but I'd draw the line at North and South, I wouldn't go beyond the Roaring Forties for that I'd need a bigger boat.

Even a couple can do it on a tiny boat as another Swede proved some years ago (IIRC the different designs were named "Bris").

He (Sven Lundin, now renamed Sven Yrvind) cruised those small boats for several years, he even rounded Cape horn in one. He's on his way soon again and he's well past 70 now. Link to his webpage: Welcome to Sven Yrvind- the official website

This guy, another Swede sailed from Sweden to the Caribbean and up to Florida, put his boat on a trailer, bought a $200 car and used the boat as a caravan while he drove to Canada. He then let the boat sit in a friends back yard for seven years, before he returned and hauled it back dow to Florida and sailed her back to Sweden. Web page: Arrandir and Sebastian

I think CarinaPDX sums it up quite well, although this year there was a Sadler 29 which completed in a respectable time.

However the ARC is quite expensive not only to enter but fit out with equipment, so those who are in a financial possition to do it are also likely to be able to afford a larger craft. Those sailing on a limited budget and therefore probably a smaller yacht are likely to be solo crossing. ARC limit was given at 27 feet when we attended there forum brief in September.

Interesting response from our insurers Pantaenious when enquired about the ARC in 31 foot yacht with husband and wife crew. They said we would rather see a well equipped 31 foot yacht that could easily be handled by one person on watch instead of 45 foot yacht which might be too much for a small crew. Taking part in the ARC would assist in getting insurance cover because the checks you need to pass before setting off.

I did some safety boat work for a group of ocean rowers a couple of years ago. Now they are completely barking mad. A 21 foot boat with just 150L of water and a hand pumped watermaker for the rest. A single burner stove to boil all the water and make a flask for the following 24 hours. Oh and an average cruising speed of 2 knots Makes our 31 foot yacht look palatial.

Scotland to the Canaries in summer quite "do able" with good forecasts. In the depths of winter, no chance.